Overview of constraints
Constraint modules enable information architects to restrict the content models or attributes of OASIS-defined DITA grammars. A constraint is a simplification of an XML grammar such that any instance that conforms to the constrained grammar also will conform to the original grammar.
A constraint module can perform the following functions:
- Restrict the content model for an element
- Constraint modules can modify content models by removing optional elements, making optional
elements required, or requiring unordered elements to occur in a specific sequence. Constraint
modules cannot make required elements optional or change the order of element occurrence for
ordered elements.
For example, a constraint for
<topic>can require<shortdesc>, can remove<abstract>, and can require that the first child of<body>be<p>. A constraint cannot allow<shortdesc>to follow<prolog>, because the content model for<topic>requires that<shortdesc>precedes<prolog>. - Restrict the attributes that are available on an element
-
Constraint modules can restrict the attributes that are available on an element. They also can limit the set of permissible values for an attribute.
For example, a constraint for
<note>can limit the set of allowed values for the@typeattribute to "note" and "tip". It also can omit the@othertypeattribute, since it is needed only when the value of the@typeattribute is "other". - Restrict the elements that are available in a domain
-
Constraint modules can restrict the set of extension elements that are provided in a domain. They also can restrict the content models for the extension elements.
For example, a constraint on the programming domain can reduce the list of included extension elements to
<codeph>and<codeblock>. - Replace base elements with domain extensions
- Constraint modules can replace base element types with the domain-provided extension
elements.
For example, a constraint module can replace the
<ph>element with the domain-provided elements, making<ph>unavailable.